1. Field of the Invention
The present invention generally relates to digital radar landmass simulation (DRLMS) and, more particularly, to a real time three-dimensional, high-resolution radar weather simulation with adjustable parameters, as a modular addition to a multi-channel digital landmass simulator for flight simulations. More specifically, the invention relates to certain techniques for data compression/decompression of weather patterns, for weather attenuation and backscattering and weather expansion, and for superposition on a ground map.
2. Description of the Prior Art
Computer image generation (CIG) is used in visual training simulators which present scenes to an observer or trainee to allow the observer to practice some task, such as flying an airplane. In a flight simulator, for example, a three-dimensional model of the desired "gaming area" is prepared and stored on magnetic disk or similar bulk storage media. The visual simulator combines an image generator with an electro-optical display system such as a cathode ray tube (CRT) or similar display. The image generator reads in blocks of three-dimensional data from the disk and transforms this data into two-dimensional scene descriptions. The two-dimensional data are converted to analog video that is presented to the operator or trainee via the display. The generated imagery is meant to be representative of the true scenes that the operator would see if the operator were actually performing the task being simulated. The generation of the display images is said to be in "real time" which is normally taken to mean 30 frames per second, as in the U.S. television standard. CIG systems are described in detail in the book entitled Computer Image Generation edited by Bruce J. Schacter and published by Wiley-Interscience (1983).
Radar simulation is an important tool for the training of pilots. There has been much progress in radar in the recent years in terms of higher resolution. Typically, the radar is used for storm avoidance, obstacle avoidance, navigation in poor weather, and target acquisition, among other things. Accordingly, a digital radar land mass simulator (DRLMS) has to be able to process the ever increasing amount of landmass data in real time. Data compression and data retrieval have become a critial area where new techniques and hardware are needed to be developed that are cost effective and support the higher throughput rate required for DRLMS.
U.S. Pat. No. 3,769,442 to Heartz discloses a digital radar landmass simulator wherein the cultural features and prominent terrain features such as ridges and valleys are described by means of a sequence of connected edges. Each edge is defined by the two end positions in x,y,z coordinates and the direction. This edge information is stored in an on-line memory. The real time hardware then interpolates between the end points of the data along the edge. This technique can generate good data compression when the edges are long. This technique is only for the encoding of prominent terrain features and does not apply to the compression of a geographical area at a resolution of 30 meters for level II and 100 meters for level I. In a later patent, U.S. Pat. No. 4,017,985, Heartz discloses a system wherein the terrain is fitted with a number of faces enclosed by edges. The terrain along a radial sweep is calculated by its intersection with the faces. For large faces, the compression ratio is high. However, for high resolution data bases, when the number of faces approaches the number of display pixel elements, the data stored for the faces may exceed the data otherwise stored for each pixel, and the advantage of this compression technique diminishes.
Others have described data compression and reconstruction techniques in digital moving map displays. The requirements for data retrieval, compression and reconstruction are similar between digital moving map displays and DRLMS. As one example, U.S. Pat. No. 4,520,506 to Chan et al. describes a modified boundary/footprint approach for the compression of culture features. The scheme is that the compression of culture including linear and area data, is based upon a line generating technique, knowing the starting and the end point data and the gradient in between. To reconstruct an area knowing the information describing the edges enclosing it, a scan line data can be filled in knowing the end point values defined by the intersections of the scan line with the left and right edges of an area. The area, line and point data are reconstructed in descending priority. Again, the compression technique is to encode the feature data in terms of the end points of an edge. Large compression can be achieved when the lines are long and the surfaces are large.
Weather simulation has two components, the simulation of the backscattering of radar return of the weather mass itself and the attenuation of terrain by the weather. A typical weather radar simulator simulates the weather indicator display of an aircraft; i.e., the backscattering of a cloud formation only without the terrain return. The antenna shape for weather radar is usually a pencil beam, whereas a ground map radar has a cosecant square shape beam. A typical weather radar would have different colors indicative of precipitation thickness. See for example U.S. Pat. No. 4,667,199 to Roberts. A digital radar landmass simulator, with weather simulation, on the other hand has both components; the backscattering of weather and its attenuation of terrain/target.
A sophisticated weather simulation in a digital radar landmass simulator (DRLMS) or in weather radar for simulating a three-dimensional weather mass with multiple radar beam paths cannot be faulted in performance, but the cost is formidably high and difficult to achieve in real time and is sometimes considered out of proportion to both the training it offers and to the overall cost of an aircraft simulator. Several ways of modeling weather have appeared in the industry.
The three-dimensional weather masses simulated in DRLMS in the market today are modeled as simple polygons or geometric objects (e.g., cylinders). Though the processing was done in real time, the weather appeared to be artificial. Others, including both DRLMS and weather radar, modeled the weather the same as terrain with reflectivity and heights. Therefore, the weather does not have a bottom and no adjustments for heights. The simulated weather mass reaches from the top of the cloud mass to the top of the terrain. There is no gap between the terrain top and the cloud. A weather radar simulator disclosed in U.S. Pat. No. 4,493,647 to Cowdrey showed the simulation of the radar return of a weather mass composed of maps of weather cells with intensity, bottom and top. However, in the weather radar simulation, the interaction of weather with terrain (weather shadowing terrain) as required by DRLMS, was ignored. Furthermore, the weather precipitation, bottom and height, was not adjustable.
A high fidelity real time multi-channel digital radar landmass simulator is disclosed in my prior U.S. Pat. No. 4,890,249. This simulator has a modular architecture to simulate radar for simple shore-line applications to a full high fidelity air-borne radar simulator. However, the weather effects, i.e., weather backscattering, the attenuation of targets and ground map by weather mass, was not simulated. What is needed is to simulate ground map radar with the modifications by weather environmental effects.